2019 VJC H2 Chem Prelim P2 QP

 VJC 2019 9729/02/PRELIM/19 [Turn over 2 1 (a) An unknown solid sample, with a mass of 1.50 g, contains three sodium salts, NaC l, NaClO3 and NaNO3. The sample was completely dissolved in water and diluted in a 250 cm3 volumetric flask to obtain solution L. In one experiment, a 50 cm3 portion of solution L was reacted with excess silver nitrate solution. The AgC l precipitate formed was removed by filtration, dried and weighed. The AgCl precipitate was found to have a mass of 0.240 g. In another experiment, a gas was bubbled into a new 50.0 cm3 portion of solution L to convert ClO3 to C l. Excess silver nitrate was then added to the resulting mixture. The AgC l precipitated formed was treated similarly as before and found to have a mass of 0.285 g. (i) Determine the amount of ClO3- ions present in 50 cm3 of solution L. [2] (ii) Hence, determine the percentage by mass of NaClO3 present in the original solid sample. [2] (b) Suggest whether NaCl or NaClO3 has a lower melting point. Explain your answer. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. [2]

 VJC 2019 9729/02/PRELIM/19 [Turn over 3 (c) (i) Draw a dot-and-cross diagram of the ClO3– anion. [1] (ii) Chlorine forms a number of oxides, one of which is Cl2O7. Cl2O7 is a symmetrical molecule, with a central oxygen atom bonded to two chlorine atoms. Draw the structure of Cl2O7. [1] (iii) The bonds in Cl2O7 exhibit two different bond lengths, namely, 0.141 nm and 0.171 nm. However, all the bonds in the ClO3– ion have the same bond length of 0.149 nm. Suggest why the bond length in ClO3– is intermediate between those present in Cl2O7. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [2]

 VJC 2019 9729/02/PRELIM/19 [Turn over 4 (iv) The bond angle formed between the central oxygen atom and its two surrounding chlorine atoms is larger than expected with a value of 118.6o. Suggest a possible reason why. …………………………………………………………………………………………… …………………………………………………………………………………………… [1] [Total: 11] 2 (a) Use of the Data Booklet is necessary for this part of the question. The sub–atomic makeup of certain ions formed from isotopes of cobalt and lead is given below: number of neutrons number of electrons Co ion 33 25 Pb ion 122 In a particular experimental set–up, a beam containing the above ions of cobalt was passed through an electric field and was deflected by an angle of +10.2o. Under identical conditions, another beam containing the above ions of lead was deflected by an angle of +6.0o. What is the overall charge of the lead ions? [2]

 VJC 2019 9729/02/PRELIM/19 [Turn over 5 (b) Many transition metals and their complexes are paramagnetic. Paramagnetism is a property of a substance which allows it to be weakly attracted to a magnet. This property is due to the presence of unpaired electrons in the substance. CoF63– and [Co(NH 3)6]3+ are both complexes with cobalt in the +3 oxidation state. However, only CoF63– displays paramagnetism while [Co(NH 3)6]3+ does not. This is determined by whether the electronic configuration of the Co 3+ ion in the complex displays a “high spin” or a “low spin” state. The following diagram shows how the d –orbitals are spl it in an octahedral environment. In a ‘high spin’ state, the electrons occupy all the d –orbitals singly, before starting to pair up in the lower energy d–orbitals. In a ‘low spin’ state, the lower energy d –orbitals are filled first, by pairing up if necessary, before the higher energy d–orbitals are used. (i) Draw the shapes of the dxy and dz2 orbitals. Label your drawings clearly. [2]

 VJC 2019 9729/02/PRELIM/19 [Turn over 6 (ii) Using ↑ or ↓ to represent electrons, show, on the two diagrams below, the electronic distribution of a Co3+ ion in a high spin state, and in a low spin state. Hence, identify the cobalt complex that corresponds to each particular spin state. High spin Low spin Complex: ………………………. ………………………. [2] (iii) Suggest why electrons usually prefer to occupy orbitals singly rather than in pairs. …………………………………………………………………………………………… …………………………………………………………………………………………… [1] (iv) Using the explanation in (b)(iii), together with the information given above, state and explain which of the two cobalt complexes contains the larger energy gap, E, between its d orbitals. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [1] [Total: 8]

 VJC 2019 9729/02/PRELIM/19 [Turn over 7 3 Frederick Thomas Trouton was an Irish physicist who observed a relationship between boiling poin ts and enthalpy changes of vaporisation after studying many liquids. He published his findings and formulated Trouton’s rule which states that the molar entropy of vaporisation, ΔSvap, for most liquids is about 85 J K1 mol1. The boiling points and enthalpy changes of vaporisation of several organic liquids are as follows: substance boiling point / °C ΔHvap / kJ mol1 ΔSvap / J K1 mol1 propanone, (CH3)2CO 56.1 29.1 dimethyl ether, (CH3)2O 24.8 21.5 86.6 ethanol, CH3CH2OH 78.4 38.6 octane, CH3(CH2)6CH3 125.6 34.4 86.3 pyridine, 115.3 35.1 90.4 (a) Explain the term entropy and why ΔSvap at constant pressure is positive. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… [2] (b) Complete the above table by calculating the ΔSvap for propanone and ethanol. Show your working for the calculation of ΔSvap for propanone. [2]

 VJC 2019 9729/02/PRELIM/19 [Turn over 8 (c) By considering the structure and bonding of the various substances in this question, state and explain the relationship between strength of intermolecular forces of attraction and the entropy of a substance in the liquid state. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… [2] (d) (i) With the aid of a diagram, show and label the bonding between two methanoic acid molecules in the gaseous phase. [1] (ii) Hence, predict if ΔSvap for methanoic acid is more or less positive than predicted by Trouton’s rule. Explain your reasoning. [No marks will be awarded if a prediction has no reasoning.] ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………………………... ……………………………………………………………………………